In abrasive blasting, air flowing at a high volume and velocity carries abrasive particles with it. The abrasive particles function as tiny projectiles which chip or abrade a workpiece. The substrate of the workpiece may be worn away by the blast. Abrasive blasting and/or a high pressure mixer can be used to remove scale, rust, corrosion and other materials from a surface. Old or dead paint on a surface can also be removed. It is literally chipped away in tiny pieces by the blast.
Abrasive blast apparatus is fairly old. It is a very successful technique of preparing a metal surface for recoating, for removing rust, etc. The present invention enhances a blast apparatus by suppressing the cloud of dust that is formed.
Abrasive blasting is typically accomplished through the use of graded abrasive particles of a selected composition. The blast particles that are chosen are typically chosen with two factors in view, one being the size of the particles and the other being the physical properties of the particles. For instance, the particles can be screened and thereby selected to obtain large or relatively small blast particles. A selection of particle with many sharp corners created by crushing enhances the abrasive action. The life and durability of the particles is in part determined by the physical properties obtained from various types of abrasives. In the application of abrasive blast to a workpiece, the sand hits it quite sharply, and the particles are often broken. When they are broken, they break into tiny pieces which are generally described as dust. The pieces are quite small, sufficiently small that they can be airborne upwardly, in effect, forming a large cloud of dust. The dust is scattered far and wide as a result of the sideblast from the diverted stream of air associated with blasting. This scatters the dust and keeps it stirred, dissipating it over a wide area and carrying all types of chemicals and particles chiseled from the surface.
The present invention has as one of its objects the reduction of dust dissipation. The dust at least constitutes a health hazard. The cloud of dust requires respirators on operators in many circumstances. It can also create an explosive atmosphere in some instances. It requires suppression through the use of hoods, special enclosures and the like, all of which are very expensive.
With the foregoing problem in view, the present invention has as one object the incorporation of a means which adds water in a controlled quantity to all particles of the sandblast flow. The water is in large part atomized as it flows with the abrasive blast emerging from the nozzle. In any case, it moistens the stream of air which carries the blast particles, and the particles are slightly dampened. It does not impede the performance of the abrasive blasting procedure, itself; it does, however, moisten the dust and cause the dust to settle more rapidly. Indeed, by controlling the rate of flow of water, the dust can be caused to settle along with the heavier particles reboudning from the workpiece.
One advantage of the present invention is the ability of the water to carry waterborne inhibitors. Rust, neutralizers and oxidation inhibitors can be added to the water and thereby introduced into the blast media stream. This can be beneficial either to the rebounding blast particles or to the surface of the workpiece, depending on the requirements. As an example, the induction of water soluble inhibitors, coupled with the electrostatic charging of particles in the abrasive blast nozzle, can place an ion-attached inhibitor coating on the workpiece.
Another feature of the present invention is the extent of mixing. In particular, mixing is achieved in the nozzle, itself. Mixing is in large part dependent on scale factors, including the length of the nozzle, the position of the manifold, the ports which introduce water, the flow rate and the like. The turbulence that occurs within the nozzle is in large part dependent on the design of the nozzle. Through the use of design factors which yield a high turbulence mixing in the nozzle, it is possible to accomplish good dispersion of the blast particles through the nozzle and also to disperse the water that is introduced. In light of the high flow rates which occur, it is possible to obtain turbulence with a Reynolds number of 250,000 and up. This thoroughly wets each blast particle which flows through the nozzle and will fairly well moisten the blanket of moving air which carries the particles. The homogenizer effect can be accomplished at much less cost than present types.
One advantage of the apparatus of this disclosure is the incorporation of a valve means which varies the flow of water. The flow of water can be tailored to meet specific needs. For example, the device may not need water at times when the air pump is inoperative and, yet, no particles are introduced into the stream of air.